1. Field of the Invention
The invention relates to linear voltage regulators, and in particular to such regulators with series pass power semiconductor output stages.
2. Description of Related Art
In a linear voltage regulator with a series pass output stage, power dissipation in the output stage varies linearly with changes in current demands supplied to a load by the regulator and with the voltage drop in the output stage. In such a voltage regulator, the output stage includes a variable impedance connected in series with the load. Typically, this variable impedance comprises one or more semiconductor devices which must dissipate substantial power at high load currents. This necessitates the use of high power semiconductor devices mounted on large heat sinks. These heat sinks occupy space that could be better used and are often more expensive than the power semiconductors themselves.
To facilitate comparison of several output stage circuits, a typical specification for a television voltage regulator of the above described type will be utilized. This exemplary specification requires the capability of supplying to a load a regulated voltage of 123 VDC over a load current range of 0.21 amp to 0.78 amp from an unregulated input voltage V.sub.in which can vary from 130 VDC to 165 VDC. The power dissipation in the output stage of this linear voltage regulator at the maximum input voltage and maximum load current is approximately 33 watts. The semiconductor devices and heat sink(s) needed to dissipate this amount of energy would be commercially prohibitive.
FIG. 1a illustrates a known linear voltage regulator in which the power dissipated in the semiconductive impedance of the output stage (enclosed in the dashed line box) is reduced by incorporating a shunt resistor R.sub.s in the output stage. This shunt resistor is electrically connected in parallel with a power transistor Q10 of the output stage to reduce the percentage of load current I.sub.L which must pass through the output impedance of the transistor. The magnitude of this output impedance is controlled by well known control circuitry such as the feedback circuit illustrated in the figure. Briefly, this circuit includes an error amplifier Al having a first input to which a reference voltage V.sub.ref is supplied, a second input electrically connected to a node of a resistive divider circuit (R2,R4) for sensing the regulated output voltage V.sub.out, and an output electrically connected to the base of transistor Q10 for controlling the output impedance of this transistor. The reference voltage V.sub.ref is supplied by a zener diode circuit (R1, D.sub.z).
The currents passing through the shunt resistor and the transistor at different load currents are illustrated in FIG. 1b. To reduce power dissipation in the transistor, the current through the shunt resistor is made as large as possible within the operating limitations of the voltage regulator. In the illustrated circuit, however, the shunt current may not be made larger than the specified minimum load current of 0.21 amp, or the transistor will cut off above this load current and the output voltage V.sub.out will become unregulated. This output stage is not capable of regulating below 0.21 amp.
The power dissipated in the shunt resistor and the transistor over the specified load current range is illustrated in FIG. 1c. At the maximum input voltage (165 VDC) and maximum load current (0.78 amp) the shunt resistor dissipates only about 9 watts while the transistor dissipates about 24 watts. Thus, at maximum load the transistor dissipates over 70% of the power dissipated in the output stage. FIG. 2a illustrates a linear voltage regulator including in its output stage a shunt resistor R.sub.s ' and first and second parallel transistor circuits comprising a first transistor / emitter resistor combination Q21 / R21 and a second transistor / emitter resistor combination Q22 / R22, respectively. This output stage functions similarly to that of FIG. 1a, except that the two transistors share the semiconductor power dissipation.
The currents passing through the shunt resistor and the two transistors at different magnitudes of load current I.sub.L are illustrated in FIG. 2b. As in the single transistor circuit arrangement of FIG. 1a, the current through the shunt resistor may not be made larger than the specified minimum load current of 0.21 amp, or the transistor will cut off above the minimum load current and the output voltage V.sub.out will become unregulated. Thus, this output stage is also incapable of operating as a regulator at load currents below 0.21 amp. The first transistor Q21 conducts current throughout the specified load current range, but the second transistor Q22 conducts current only above that value of load current at which the current through resistor R21 develops a voltage drop sufficient to forward bias the base-emitter junction of the second transistor.
The power dissipated in the shunt resistor and the two transistor circuits is illustrated in FIG. 2c. Note that at the maximum input voltage (165 VDC) and load current (0.78 amp) the shunt resistor still dissipates only about 9 watts, while the two transistor circuits collectively dissipate about 24 watts. The primary advantage of this output stage circuit arrangement over that of FIG. 1 is that each of the transistors and their respective heat sinks may have lower power dissipation ratings than that of the single transistor output stage. However, this offers no cost advantage over the single transistor arrangement.